If you are talking about the 12 volt, you could jump start it or put a battery charger on it. If you are talking about the big battery, hello Toyota service department.
The 12 volt turns on the computers and the big battery, the big battery then will start the engine if needed.
the key fob has a button battery. if it dies, you can hold the fob up to the start button and an rfid chip will allow you to start the car. if the 12v dies, you can jump it like a regular car, but there are precautions. if the hybrid battery dies, you sool.